Flapping when stepping up is something I’ve only observed in Ducky when he was clipped as a baby. Ducky used to be really clumsy, breaking all his tail feathers and slipping while climbing in his cage. But after Ducky went through his first molt, he got all his tail and flight feathers back and became a graceful flyer who isn’t clumsy at all and hasn’t broken a feather since. I think it has to do with a combination of being a baby, having no tail feathers, and being clipped. Tails are used for balance, so combine the lack of tail feathers with a bad clip and pure baby clumsiness and you have a clumsy bird. Now, I can even run around the room with Ducky perched on my shoulder and he hardly budges (except he may fly off in annoyance). Also, birds who had their nails trimmed recently tend to slip more often as they use their nails to grip things.
Kermit also does the walking upside down on top of his cage. Even though he has two nails missing (the two back ones on one foot), you could hardly call him clumsy. He has never broken a single feather in his life.